Typical lithographic processes involve formation of a patterned resist layer by patternwise exposing a radiation-sensitive resist to an imaging radiation. The image is subsequently developed by contacting the exposed resist layer with a material (typically an aqueous alkaline developer) to selectively remove portions of the resist layer, thereby obtaining the desired pattern. The pattern so obtained is subsequently transferred to an underlying material by etching the material through the openings of the patterned resist layer. After the transfer is complete, the remaining resist layer is then removed.
The resolution capability of lithographic processes is generally a function of the wavelength of imaging radiation, the quality of the optics in the exposure tool and the thickness of the imaging layer. As the thickness of the imaging resist layer increases, the resolution capability decreases. Thinning of a conventional single layer resist to improve resolution generally results in compromise of the etch resistance of the resist which is needed to transfer the desired image to the underlying material layer.
In order to obtain the resolution enhancement benefit of thinner imaging layers without compromising the etch resistance of the resist, bilayer lithographic processes have been developed which feature a thin imaging resist layer (typically a silicon-containing resist) disposed on a thicker planarizing layer. The thinness of the imaging resist layer allows a pattern to be imparted to it with high resolution. This pattern is then transferred to the thicker planarizing layer. The patterned planarizing layer is then utilized as an etch mask to etch the substrate. Since the pattern planarizing layer may be substantially thicker than the imaging layer, it provides greater etch resistance. Hence, the use of a bilayer photoresist provides resolution enhancement without an associated decrease in etch resistance.